PROJECT SUMMARY Prenatal exposures to environmental chemicals have been shown to cause adverse later life health effects, often involving disorders of reproductive dysfunction. The overall goal of this research is to understand the mechanisms governing accumulation of environmental chemicals in the embryo, so that we can predict and mitigate the negative effects of these exposures. In this proposal, we address two key questions with regard to xenobiotic accumulation in the embryo, with a specific focus on the role of xenobiotic transporters during primordial germ cell (PGC) formation. First, we ask how the program of development leads to changes in xenobiotic transporter expression, and thus generates windows of susceptibility or resistance to xenobiotic accumulation. Second, we ask how real-world chemical mixtures, containing both substrates and inhibitors of transporters, impact the efficacy of this conserved, protective system. Aim 1 uses a powerful in vitro molecular evolution technology to rapidly evolve, validate, and use antibody-like binders called nanobodies to characterize xenobiotic transporter proteins in human PGC-like cells (PGLCs) and in model organism embryos (sea urchin and zebrafish). Aim 2 applies biochemical and cellular approaches to determine relevant environmental ligands of human and model system xenobiotic transporters, and takes advantage of a powerful molecular structure determination pipeline to dissect the molecular mechanisms of these interactions. Aim 3 uses models and molecular targets from Aims 1 and 2 to test the hypothesis that PGCs are vulnerable to the interfering effects of environmental chemicals on the transporter defense system, and that disruption of this system leads to decreased reproductive fitness after xenobiotic challenge. This results will provide new insights into how environmental and developmental factors act in combination to govern the susceptibility of the nascent embryonic germ line to teratogens.